1. Field of the Invention
This invention generally relates to cable television (CATV) communication systems. More particularly, the invention relates to a dual conversion CATV terminal which selectively relocates the high intermediate frequency (HI-IF), or selects a specific HI-IF frequency depending on the CATV frequency plan and the desired channel.
2. Description of Related Art
Most current CATV settop terminals use dual or multiple conversion tuners in order to achieve distortion performance and channel flatness of an acceptable quality. Single conversion tuners, widely used in most television receivers, fail to achieve optimum performance with a full spectrum of channels as is typically provided from a CATV network operator. Since single conversion tuners must operate with very large signal level differences between channels, adjacent channels are left empty and the total number of broadcast channels is relatively low when compared to a CATV system.
The basic structure of a typical dual conversion tuner includes a 47 to 860 MHz bandwidth input coupled to a heterodyne mixer or modulator which upconverts the desired input channel to a higher frequency. By using a HI-IF filter, image rejection, channel flatness, and channel loading to the second mixer is improved. This upconverted output is filtered by the HI-IF filter and sent to a second mixer which downconverts the signal to the desired output intermediate frequency (IF), which is typically between 41 and 47 MHz for a 6 MHz wide U.S. National Television System Committee (NTSC) or quadrature amplitude modulated (QAM) channel. The lower IF permits the use of economical surface acoustic wave (SAW) filters. Each mixer requires an oscillator which generates a specific frequency for the desired heterodyne output. However, the oscillator and mixer generate harmonics of the fundamental oscillator frequency which may mix with harmonics of the other oscillator within any active component, such as a mixer, oscillator, or an amplifier. When these harmonics mix together at the correct frequency, a particular "Oscillator Difference Beat" will fall within the output IF band interfering with the signal.
The creation of beats is dependent solely upon the difference between the two local oscillators and their harmonics and is independent of the input RF signal. Accordingly, since the first local oscillator varies in frequency when tuning to a specific channel, the situation is dynamic. As cable television systems increase their transmission bandwidth to frequencies of 750 MHz or higher, the potential for oscillator difference beats also increases. The use of integrated circuits and smaller tuner chassis makes the oscillator difference beats more difficult to prevent using traditional methods.
There have been attempts to minimize the adverse effects of oscillator difference beats. One of the most widely used methods is to mechanically shield the upconverter section from the downconverter section. Typically, a single or double-walled metal partition divides the printed circuit boards of each section. Covers are also used to enclose the chassis sections. A capacitive feed-through terminal is sometimes used to improve isolation between the two sections. However, mechanical isolation requires a minimum metal thickness of the chassis and compressional forces between the chassis and covers to adequately isolate the sections from each other. This frustrates attempts to reduce the size of tuner modules while directly adding to the cost.
A second approach is to select a HI-IF frequency which minimizes the number and/or severity of oscillator difference beats which fall within the IF band for a particular cable system or channel frequency plan standard. However, this requires knowledge of a particular channel map for a given cable system or channel standard. This solution is impractical in view of three widely used U.S. CATV systems which use different frequency plans. As described in the EIA-542 standard, there are standard (STD), incrementally related carriers (IRC), and harmonically related carriers (HRC). Additionally, many European systems do not use a standard channel map.
Accordingly, there exists a need for an inexpensive method to reduce oscillator difference beats in dual conversion CATV settop terminals regardless of the frequency plan used by the cable operator.